This invention relates to a photographic element and in particular a photographic element comprising at least one layer sensitized to blue light.
The use of two or more spectral sensitizing dyes that respond to a discrete spectral region (red, green or blue) when adsorbed onto the surface of silver halide emulsion grains is well known in the literature. The primary benefit derived from the use of more than one spectral dye is the improvement in color reproduction and saturation in the final recorded image. In the red and green region of the visible spectrum, where the photon flux output of a daylight 5500xc2x0 K. light source is relatively flat, the use of two or more spectral sensitizing dyes does not result in an apparent emulsion speed loss. In fact, speed gains are often observed. Since the total amount of sensitizing dye that can be accommodated on the emulsion grain surface is fixed by the molar surface area of the grain (and the molecular area of the dye) the presence of a second shorter wavelength dye absorbing light in a region of equal photon flux to that of the complimentary longer wavelength dye compensates (or may even overcompensate) for the reduced amount of first dye that is used.
However, in the blue region of the spectrum (400 to 500 nm), the flux of light from typical daylight light sources (and tungsten light sources) increases with wavelength (FIG. 1). Combination of a shorter wavelength absorbing dye (e.g. 440 nm) with a longer wavelength absorbing dye (e.g. 470) will result in less light being absorbed by the silver halide emulsion compared to the case of the longer wavelength dye used alone. This will result in a real speed loss-typically a magnitude of 0.1 to 0.2 log E (26 to 37%). In this situation, emulsion speed is being sacrificed for color reproduction.
The need to use a shorter blue wavelength dye in combination with longer wavelength blue dye to enable faithful color reproduction is particularly relevant for tabular emulsions and for 3 dimensional morphology (3D) emulsions that are predominantly AgCl. As used herein, the term xe2x80x9c3D grainxe2x80x9d refers to non-tabular morphologies, for example cubes, octahedra, rods and spherical grains, and to tabular grains having an aspect ratio of less than 2. AgBr or AgBrI 3D emulsions absorb a substantial amount of blue light in the volume of the emulsion grain and sensitizing dyes can only add additional light absorption in the longer blue wavelengths. However, for emulsions of tabular morphology or high chloride 3D emulsions, the major portion of blue light absorption must be provided by sensitizing dyes since only a small amount of this light absorption is contributed by the volume light absorption of these grains. For tabular grains, the amount of this volume light absorption is dependent on the iodide content and the thickness of the tabular emulsion grain. Thin tabular emulsions, tabular emulsions with little or no iodide content, and AgCl emulsions with little or no bromide or iodide content are particularly deficient in blue light absorption and thus especially require the combination of shorter wavelength and longer wavelength absorbing blue dyes to give faithful color reproduction. Thin tabular emulsions have advantages related to savings in silver usage. AgBrI tabular emulsions with low iodide, AgBr tabular emulsions, and high chloride emulsions of any morphology all have advantages related to more rapid photographic processing with lower replenishment rates. Thus, it is desirable to have a technology that allows these types of emulsions to be used as blue sensitive layers without compromising speed or color reproduction.
We have found that the use of fragmentable electron donor (FED) compounds in conjunction with a broad blue spectral sensitization (i.e. two dyes, one absorbing near 440 nm and the other near 470 nm) can overcome the speed loss associated with this type of blue sensitization.
One aspect of this invention comprises a photographic element comprising a support and at least one blue sensitive silver halide emulsion layer containing a tabular grain silver halide emulsion spectrally sensitized with at least one dye providing a peak sensitization between 446 and 500 nm and at least one dye providing a peak sensitization between 400 and 445 nm and additionally sensitized with a fragmentable electron donor of the formula: Xxe2x80x94Yxe2x80x2 or a an electron donor that contains an xe2x80x94XYxe2x80x2 moiety;
wherein
X is an electron donor moiety, Yxe2x80x2 is a leaving proton H or a leaving group Y, with the proviso that if Yxe2x80x2 is H a base, xcex2xe2x88x92, is covalently linked directly or indirectly to X and wherein:
1) Xxe2x80x94Yxe2x80x2 has an oxidation potential between 0 and about 1.4 V; and
2) the oxidized form of Xxe2x80x94Yxe2x80x2 fragments to give the radical Xxe2x80xa2 and the leaving fragment Yxe2x80x2; and, optionally,
3) the radical Xxe2x80xa2 has an oxidation potential xe2x89xa6xe2x88x920.7 V (that is, equal to or more negative than about xe2x88x920.7 V).
Another aspect of this invention comprises a photographic element comprising a support and at least one blue sensitive silver halide emulsion layer containing a silver halide emulsion in which the halide content is at least about 50% chloride and no more than 5% iodide, wherein the emulsion is spectrally sensitized with at least one dye providing a peak sensitization between 446 and 500 nm and at least one dye providing a peak sensitization between 400 and 445 nm and is additionally sensitized with a fragmentable electron donor of the formula: Xxe2x80x94Yxe2x80x2. or an electron donor which contains an xe2x80x94XYxe2x80x2 moiety;
wherein
X is an electron donor moiety, Yxe2x80x2 is a leaving proton H or a leaving group Y, with the proviso that if Yxe2x80x2 is H a base, xcex2xe2x88x92, is covalently linked directly or indirectly to X. and wherein:
1) Xxe2x80x94Yxe2x80x2 has an oxidation potential between 0 and about 1.4 V; and
2) the oxidized form of Xxe2x80x94Yxe2x80x2 fragments to give the radical Xxe2x80xa2 and the leaving fragment Yxe2x80x2; and, optionally,
3) the radical Xxe2x80xa2 has an oxidation potential xe2x89xa6xe2x88x920.7V (that is, equal to or more negative than about xe2x88x920.7V).